Field
This invention relates generally to a system for tracking a beam of light and, more particularly, a system for tracking a beam of light that includes one or more mirrors controlled by drivers and actuators, where one mirror may be a high speed tracking mirror and another mirror may be a nutator mirror, and where each mirror is driven by a high voltage position drive circuit that provides a low current, low frequency, or near DC, position drive signal to an actuator and a low voltage drive circuit that provides a high current, high frequency drive signal to the actuator.
Discussion
Communications systems are known in the art that transmit data and other information from a transmitter to a receiver on a modulated beam of light. A sensor in the receiver receives the light beam and converts it to an electrical signal for processing. In order to receive the beam in an effective manner to collect the data therefrom, and at greater distances, it is necessary to orient the optics of the sensor toward the transmitter. Because for some of these types of communications systems there is relative movement between the transmitter and the receiver, it is often necessary for the receiver to actively track the beam. Although the best reception of the beam would occur if the light beam is pointed directly at the sensor, it is difficult to track the beam with that level of accuracy.
One technique for tracking a beam of light is to employ a sensor that has the capability to determine the degree and direction of misalignment of the beam and a feedback control system that uses the misalignment estimate to operate a tracking mechanism that alters the beam direction. The bandwidth of the control system used for this purpose must be sufficient to track the relative motion of the beam and to suppress high frequency vibrational disturbances on the optical system, and further, the tracking mechanism and its associated drive circuit must have a bandwidth on the order of 10 times greater than the control system bandwidth to avoid instability. However, for the various actuators that are employed in the art for this purpose, such as piezoelectric actuators, providing a high enough rate of the tracking mechanism requires a significant amount of power, which is an obvious drawback. For example, vibrational disturbances on these types of optical systems are typically as high as 1000 Hz, which requires bandwidth of the mechanism to be at least 10 times greater in frequency. Thus, the electronic drive circuits required to drive the tracking mechanism are often difficult to implement.
One technique for obtaining the sensor estimate of misalignment is to employ a nutator that oscillates a mirror, or other optical element, so that the beam being tracked lands offset from a center location on the sensor optics. Specifically, the mirror is rotated so that the received beam oscillates around the center location and the magnitude of the beam is observed so that if it is constant, it is known that the sensor optics is aligned with the beam. If the sensor optics is misaligned to the beam being tracked, the amplitude of the received beam will vary as the mirror is being oscillated, which provides an indication of the direction and magnitude of misalignment of the beam. In other words, the nutator causes the beam of light to be rotated in a small circle at a high speed, which is used for providing an error metric for tracking the light beam similar to a conical scan approach employed for RF antennas. By employing a nutator that imparts a slight angle of the beam directed toward the sensor optics, the modest amount of received power that is given up as a result of the nutation is compensated by the benefit of the ability to track the beam.
Actuators are employed in these types of nutators to rotate the angle of a mirror or other mechanism, which requires sophisticated control and a desired speed of rotation. Further, the speed of rotation determines the speed with which measurements of misalignment of the incoming beam can be produced. Thus, it is necessary to have a high enough rate of oscillation of the beam to produce sufficiently high speed estimates of the error that will then support a large enough control system bandwidth to suppress high frequency vibrational disturbances on the optical system. As in the case of the tracking mechanism, for the various actuators that are employed in the art for this purpose, such as piezoelectric actuators, providing a high enough oscillation rate of the mechanism requires a significant amount of power, which is an obvious drawback. For example, vibrational disturbances on these types of optical systems are typically as high as 1000 Hz, which requires nutation of the mechanism, and production of misalignment estimates, to be at least 10 times greater in frequency. Thus, the electronic drive circuits required to drive the nutator are also often difficult to implement.
Beam direction alteration for tracking and nutation to accomplish pointing the sensor optics at the optical beam being received is typically accomplished by electro-optical or electro-mechanical devices. However, electro-optical devices may have difficult constraints such as a need for polarized light and a very high drive voltage, and electro-mechanical devices consume increasing amounts of power as the frequency of operation increases, which is particularly true of devices using either piezoelectric or electrostrictive elements that are capacitive in nature. Thus, a reduction of power consumption is desired to make high frequency electro-mechanical tracking and nutation practical.